Thin film stent

ABSTRACT

A method for fabricating a stent or other medical device by creating a free standing thin film of metal.

RELATED PATENT APPLICATION

This application is a continuation of application Ser. No. 10/382,332filed Mar. 4, 2003, now U.S. Pat. No. 7,118,656, which is a continuationof application Ser. No. 09/628,079 filed Jul. 28, 2000, now U.S. Pat.No. 6,527,919, which is a continuation of application Ser. No.09/118,729 filed Jul. 17, 1998, now U.S. Pat. No. 6,096,175.

FIELD OF THE INVENTIONS

The present invention relates to treatments for vascular diseases andother diseases of body lumens, in particular to a method ofmanufacturing a stent.

BACKGROUND OF THE INVENTIONS

The inventions described below were developed with the goal of providingnew and better therapies for certain types of vascular disease for whichthe present day therapies are widely regarded as inadequate. Vasculardisease includes aneurysms which can rupture and cause hemorrhage,atherosclerosis which can cause the occlusion of the blood vessels,vascular malformation and tumors. Occlusion of the coronary arteries,for example, is a common cause of heart attack. Vessel occlusion orrupture of an aneurysm within the brain are causes of stroke. Tumors fedby intra-cranial arteries can grow within the brain to the point wherethey cause a mass effect. The mass and size of the tumor can cause astroke or the symptoms of stroke, requiring surgery for removal of thetumor or other remedial intervention.

The newly preferred therapy for coronary occlusions is placement of anexpanded metal wire-frame, called a stent, within the occluded region ofthe blood vessel to hold it open. Stents of various construction havebeen proposed, including the Palmaz-Schatz™ balloon expandable metalstent, the Wallstent self-expanding braided metal stent, the Streckerknitted metal stent, the Instent™ coil stent, the Cragg coiled stent andthe Gianturco Z stent. Stents have been proposed for treatment ofatherosclerosis in the neck, but carotid endarterectomy is still thepreferred treatment for stenosis. Most perioperative strokes are thoughtto be caused by technical errors during endarterectomy (see Becker,Should Metallic Vascular Stents Be Used To Treat CerebrovascularOcclusive Disease, 191 Radiology 309 (1994)). The same concerns militateagainst other forms of therapy such as angioplasty for treatment of thecarotid arteries. Various factors, including poor long-term patency,distal emboli causing a stroke, the potential for crushing from externalpressure, and the need for long term anti-coagulation, lead to theavoidance of certain stents in vessels smaller than the iliac arteriesor in locations susceptible to external pressure. See, for example,Hull, The Wallstent in Peripheral Vascular Disease, For Iliac Use Only,6 JVIR 884 (November-December 1995).

Stent-grafts have been proposed and used to treat aneurysms in the largeblood vessels such as the aorta, and these typically include tube graftmaterial supported by a metallic stent. These stent-grafts are designedfor use in the large blood vessels, and the various layers of stents andgrafts make them unsuitable for use in smaller blood vessels.Stent-grafts are not currently used in the coronary arteries which aretypically 3 or 4 mm in internal diameter. Rolled stents have beenproposed for use in aortic aneurysms. For example, Lane, Self ExpandingVascular Endoprosthesis for Aneurysms, U.S. Pat. No. 5,405,379 (Apr. 11,1995) suggests the use of a polypropylene sheet placed in the abdominalor thoracic aorta to bridge aneurysms. Winston, Stent Construction ofRolled Configuration, U.S. Pat. No. 5,306,294 (Apr. 26, 1994) proposes arolled sheet of stainless steel. Of similar construction are the singlelayer rolled stents such as Kreamer, Intraluminal Graft, U.S. Pat. No.4,740,207 (Apr. 26, 1988) and its reissue Re 34,327 (Jul. 27, 1993),which are expanded by balloon and include a ratchet mechanism whichprojects into the lumen of the stent. Khosravi, Ratcheting Stent, U.S.Pat. No. 5,441,155 (Aug. 15, 1995) and Sigwart, Intravascular Stent,U.S. Pat. No. 5,443,500 (Aug. 22, 1995) are other examples of rolledstents with ratcheting locking mechanisms.

Stents have not previously been used for aneurysms of the blood vesselsin the brain. The vessels in the brain likely to develop stenosis,aneurysms, AVM's and side branches requiring occlusion have diameters ofabout 1 mm to 5 mm, and can be accessed only via highly tortuous routesthrough the vascular system. The stents described below will bedelivered percutaneously, introduced into the body through the femoralartery, steered upwardly through the aorta, vena cava, carotid orvertebral artery, and into the various blood vessels of the brain.Further insertion into the brain requires passage through the highlytortuous and small diameter intra-cranial blood vessels. The Circle ofWillis, a network of blood vessels which is central to the intracranialvascular system, is characterized by numerous small arteries and bends.Passage of a stent from the internal carotid through the Circle ofWillis and into the anterior cerebral artery (for example) requires aturn of about 60° through blood vessels of only 1-5 mm in diameter.Clinically, many significant aneurysms take place in the Circle ofWillis and approaching blood vessels. The stent produced according tothe methods described herein are intended for use in such highlytortuous vessels, particularly in the Circle of Willis, the vertebraland carotid siphons and other major blood vessels of the brain. Attimes, pathologically tortuous vessels may be encountered in the deepervessels of the brain, and these vessels may be characterized by smalldiameter, by branching at angles in excess of 90° and by inaccessibilitywith guide wires larger than the standard 0.018 guide-wires. Thesepathologically tortuous vessels may also be subject to aneurysms andAVM's which can be treated with the stents produced according to themethods described below.

In order to fabricate sheet stents of extreme thinness, we have coldrolled metals such as Elgiloy, nitinol and stainless steel. Rollingappears to be effective to provide sheets of thickness down to 0.0011inches. In order to fabricate thinner sheets, we have used chemicaletching techniques to etch away even more of the sheet. This techniquehas enabled construction of sheets as thin as 0.0005″ with somewhatuniform thickness. The method of constructing the stent described below,and the stent resulting from this method, will provide rolled sheetstents made according our prior disclosures in smaller and thinnerdimensions than was previously possible. The fabrication method may beapplied to all the stents previously used and proposed in the art, withthe added advantage that stent is provided in a much thinner and/orstronger form, and may be constructed of nitinol or other metals andmaterials in a manner not previously used.

In the far-afield arts of micro-machines and microactuators, thefabrication of shape memory switches of microscopic proportions has beenproposed. The fabrication technique is called sputter deposition. Theresultant material is referred to as a thin film. The sputter-depositedfilms have been experimentally used in micro-valves and micro-grippers.Thin film sputtering processes are used in the manufacture of microchipsto lay down very small and very thin circuit lines on circuit substratessuch as silicon chips. Thin film processes are used to coat plasticarticles with decorative chrome finishes. In general, thin film sputtertechniques use high power electromagnetic fields to create energeticparticles or photons (plasma ions, ion beams, electron beam, laser beam)directed toward a target plate of the coating material to dislodgesingle atoms or molecules of the coating material onto a substrate. Thedislodged atoms or molecules condense on the substrate and adhere verystrongly to the substrate. The sputter process is usually performed atvery high temperature of one hundred to several hundred degreescentigrade, and performed within an atmosphere of very high vacuumand/or an atmosphere of an inert gas. Sputter techniques are part of abroader field of processes referred to as physical vapor deposition orPVD techniques. The PVD processes are part of the broader field of thinfilm deposition, which also includes chemical vapor deposition, andelectroplating. The key to all these processes is the placement of thesubstrate (the article to be coated) in an atmosphere or cloud of filmmolecules.

SUMMARY

The rolled sheet stents which we have proposed for use in very smallblood vessels of the brain may be constructed according to thin filmsputtering techniques. Rather than roll metals to the desired thickness,which may be on the order of several thousands of an inch and thinner,the stent is created by sputtering hot molten metal onto a moldsubstrate. This results in a stronger sheet of metal vis-à-vis therolling process. Rather than mechanically or photochemically cut thedesired perforations into the sheet, the perforations are formed duringthe sputtering process as areas which are not sputtered. When the stentis made of nitinol by thin film sputtering techniques, the resultantsheet may be cured at high temperature to provide a stent with the samepseudoelastic or shape memory properties as found in bulk preparedrolled sheets of nitinol. Thin sheets of nitinol, with a uniformthickness of 0.0002 inches and less, can be made with this technique.The specific embodiment of thin film deposition used to exemplify theinvention is RF powered physical vapor deposition. However, the varioustechniques of thin film deposition may be used.

Rolled sheet stents are preferably provided with dense perforationpatterns, as illustrated in Wallace, et al, Intracranial Stent andMethod of Use, PCT App. PCT/US97/16534. These perforation patterns havebeen made using a photochemical machining process which includes coatingthe sheet with a photoresist coating, processing the photoresist coatingto remove the coating in areas corresponding to the desiredperforations, thereby creating a partial coating which is a reverseimage of the desired perforation pattern on the sheet, etching the metalin the uncoated areas with a chemical etchant to remove the metal in theareas corresponding to the desired perforation pattern, and thenstripping the photoresist coating. This allows the creation of such thinsheets of metal without resort to mechanical cutting. By manufacturingthe stent with a sputtering technique, the perforation patterns can becreated by control of the deposition of the sputtered metal, thuseliminating the need for that the stent be subjected to photochemicalmachining process after its formation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is example of a stent to be formed with the sputtering technique

FIG. 2 is example of a stent to be formed with the sputtering technique

FIG. 3 is schematic of the process of sputtering a thin sheet ofnitinol.

FIG. 4 illustrates the processing necessary to create a thin film ofnitinol which can be released from the substrate.

FIG. 5 illustrates the processing necessary to perforate the thin filmfor use as a microporous stent.

FIG. 6 illustrates the processing depositing a perforated thin film foruse as a microporous stent.

FIG. 7 illustrates the processing depositing a perforated thin film foruse as a microporous stent.

DETAILED DESCRIPTION OF THE INVENTIONS

The thin film deposition processes may be used to fabricate a number ofmedical devices. FIGS. 1 and 2 illustrate a stent which may be formed bythe process detailed below. Although the stent may be made in manyforms, these two stents illustrate the structural elements to be formedby the sputtering process.

FIG. 1 shows a basic form of the stent 1 as formed on a sheet of stents.(The various item mentioned below are so numerous in the stent sheetthat they are called out on the drawing only in exemplary parts.) Thestents are formed in a sheet 2 of nitinol, Elgiloy™, stainless steel,plastic or other suitable material. For stents fabricated for use insmall blood vessels, the wrap length represented by transverse edge 3will be about 6-75 mm, allowing the stent to expand to diameters fromabout 1 mm to about 6 mm with approximately two to three layers ofoverlap after expansion. The bridge length represented by axial edge oraxial width 4 (or the longitudinal edge) will vary according to thewidth of the aneurysm, occlusion or other defect which must be bridgedby the stent, and may vary from 2 to 20 mm, for example. The stent isformed as a flat sheet. The stent is tempered or formed so that, whenrolled and released, it resiliently unrolls and expands to a diameter ofapproximately 1 mm to 6 mm, and provides a slight compliance mismatchwith the intra-cranial arteries which have internal diameters of about 1mm to 6 mm. Where the stent is made of nitinol, the nitinol may have anaustenitic transition temperature T_(af) of about 30° C. (slightly belowbody temperature of 37° C.), so that expansion is entirely due to theresilience of the austenite phase, or it may have a transitiontemperature slightly above body temperature so that expansion may because by application of heat, or it may be provided in a superelasticform so that expansion is due to superelastic or pseudoelastic behaviorof the film.

The stent 1 is provided with a number of perforations 5. They may beformed more specifically as a series of partial sinusoidal curves. Themultitude of perforations 5 are provided in sets 6 disposed betweenstraight radial slits 7 in the sheet. The radial slits 7 in the sheet 2leave trim tabs 8, which keep the stents attached to the sheets duringthe etching process and during other preparation processes, after whichthey are cut through to remove the finished stents from the sheet. Theperforations are crescent shaped, arcuate, but may be wave-shaped or “s”shaped in general appearance. The perforations are about 0.03-0.07inches apart radially, and 0.02634 inches apart longitudinally, and theyare about 0.00625 inches wide and 0.06102 inches long. The perforationsextend circumferentially, aligned generally with the wrap length, whilethe groups of sets 6 extend along the bridge length 4 of each stent. Thegroups of curves creates longitudinal staves of un-perforated sheetmaterial along longitudinal line such as line 10. These staves arerelatively stiff compared to the perforated area. The curves are notsignificantly inclined away from or toward the wrap length or transverseedge 3, so that a multiple number of circumferentially extending andsinusoidally winding strips of sheet material are formed within thestent. The strips are not perfectly sinusoidal, but may generally bedescribed as sinusoidal, wavy, sinuous or wiggled in a manner where thelong path of the strip is generally straight, but regularly deviatesaway from the long path. In FIG. 1, end endpoints 11 of the perforationsin respective sets are longitudinally aligned, meaning that they appearat the same point along the length 4 of the stent. The retainingperforations 12 are used to retain the stent on an insertion catheter.

The long radial or circumferential slits 7 are generally straight in thecircumferential direction, except for the curve 20 toward thelongitudinal dimension of the stent which is incorporated at theterminus 21 of each radial slit. The rounded edges thereby created inthe radial end of the stents help distribute the point force imparted onthe vessel wall and reduces the risk of vascular injury that arises withthe use of square edges. The pair of circular perforations 22 isprovided for threading a retention/release wire through the stent sheetwhile rolled upon an insertion catheter.

FIG. 2 shows the stent 1 with a different perforation scheme, withcutaways 23, 24 and 25 leaving slats or ribs 26, 27, and 28 in eventualouter layer 29, middle layer 30 and inner layer 31. The segments ofslats are separated by spines or backbones 32. The slats of each segmentare offset so that, when expanded to a roll of approximately threelayers, the three layers will overlap to form a barrier between theblood vessel wall and the inner lumen of the expanded stent.

With these structural parts of the stent in mind, the process offabricating the stent through the physical vapor deposition technique ofsputtering may be described. FIG. 3 illustrates the basic process offorming the stent with sputtering techniques. (The process is sometimesreferred to as glow discharge sputtering.) The sputter deposition may beaccomplished with several methods, all of which have the same basicmethod steps and use analogous equipment. The device for accomplishingthe sputtering includes a reaction chamber 33 which houses the target34, mounted on the target mount 35 and the substrate 36 mounted on thesubstrate holder 37. The reaction chamber is capable of holding a strongvacuum, created by operation of vacuum pumps 38 and 39. A very lowpressure Argon atmosphere is created with the injection of Argon gasthrough the argon supply line 40 (other inert gases can be used). Thepower source 41 is capable of applying high negative voltage RF or DCpower to the target 34. The intense electric field ionizes the Argon,separating the gas atoms into positively charged argon atoms andnegatively charged electrons. The positively charged Argon atoms areattracted to the negatively charged target, and accelerate toward thetarget at high energy. In this system, the target acts as the cathode,and the chamber acts as the anode, attracting the free electrons of theplasma. The substrate may be placed anywhere in the system. (Thesubstrate may be connected as the ground of the power system and act asthe anode so that is intercepts a larger portion of the sputteredmaterial.) When the argon ions hit the target 34 at high energy, theydislodge an atom or molecule of the target material from the target.This atom or molecule of the target material is propelled from thetarget material into the chamber, and some of the dislodged atoms settleonto substrate 36, where they condense on the surface. In this fashion,a film 42 of the target material is build up on the substrate, one atomat a time. Perkin Elmer sputtering systems used by various sputteringshops are typical of the systems available for accomplishing thesputtering process.

In the sputtering art, the substrate may be a silicon wafer, amicrochip, or other partially complete product. In our case, thesubstrate will be discarded at the end of the fabrication process. Thesubstrate used for stent fabrication may be made of silicon or glass,highly polished metals such as aluminum, salts such as sodium chlorideor potassium bromide, or plastic (and many other materials). Thesubstrate may be made of fluorite (CaF₂), which does not bond well totitanium, and therefore eases eventual removal of the stent from thesubstrate. The substrate may include an oxide layer and a layer of wastematerial upon which the sputtered material is deposited. The targetmaterial is generally the same as the stent (its atoms and molecules aretransferred from the target onto the substrate to form the stent). Thusa target comprising nitinol will produce a thin film of nitinol on thesubstrate upon bombardment. Where the stent is an alloy such as nitinol,the target may be pre-alloyed NiTi having the desired transitiontemperatures. The target may be comprised of two distinct targets, onetitanium and one nickel, where the alloying is accomplished bysputtering both metals in successive sputters over the substrate (themetal will alloy together upon deposition and annealing, which is adesired process in any case). Other common stent materials includingstainless steel, tantalum, Elgiloy and even polymers may be used.

Other sputtering techniques are variations on this basic technique. Inion beam sputtering, the ion beam is created using a ion beam gun. Theion beam gun directs a stream of ions at the target material to dislodgetarget atoms. The substrate is place in the chamber near the target tocollect the dislodged atoms. In this system, the target is notelectrified. The process of vacuum evaporation uses the combination ofheat and very low pressure to evaporate or sublime the target in thevicinity of the substrate, so that the evaporated target moleculescondense on the substrate. The heat may be provided thermally, throughresistive heating of the target, or by irradiating the target with laserenergy from a laser, an electron beam emitted from an electron beamemitter. Vacuum evaporation deposition is a line of sight depositiontechnique. For some stent compositions, chemical vapor deposition may beused. Chemical vapor deposition permits reaction of chamber atmospherecomponents with each other or with ejected target material to form thedeposited compound.

The process of cold sputtering can be used with a plastic substrate, inorder to form a stent on a rather inexpensive substrate made of plastic.Plastic substrates may be made of ABS plastic(acrylonitrile-butadiene-styrene copolymer), polyimide resins,polyethylene terephthalate resins and other plastics. Sputtering may beaccomplished in typical cold sputter ranges (extremely low pressureargon atmosphere, 200-600 volt applied to the cathode), with currentdensity limited to the range of about 0.2 to 5 A/dm². After the thinfilm is sputter deposited on the plastic substrate, the stent may beremoved from the plastic substrate by dissolving the plastic in asolvent.

FIG. 4 illustrates the process of forming and removing the stent fromthe substrate. In step A, the substrate 36 with a release layer 50 (anoxide layer on a silicon substrate, for example) is provided within thereaction chamber. In step B, the substrate is exposed to the sputter inthe reaction chamber and a thin film of nitinol is gradually built up onthe substrate. This film corresponds to the sheet 2, and may be about 1to 30 microns thick for use in intracranial blood vessels (and thickerfor larger vessels). The substrate 36 is typically unchanged in sizeafter deposition. The substrate, when made of silicon or metal, caninclude the thin layer of oxide on the substrate material (but someuseful substrates do not oxidize). This oxide layer may be formedintentionally on the substrate prior to sputtering, or it may merely bethe result of oxidation during normal handling of the substrate.

Referring again to FIG. 4, after the stent sheet is formed on thesubstrate, it must be removed in one piece in order to be used in thebody. Several methods may be used to remove the stent, some of whichdepend on the choice of substrate material. Where the thin film ishighly adherent, such as when sputtered on a silicon or aluminumsubstrate, a release layer of non-adherent or readily dissolvablematerial such as oxides or nitrides are placed on the substrate beforesputtering acts as a release layer. This will facilitate removal of thefilm for use as the stent sheet. The oxide layer can be formed onsilicon, for example, by exposing the silicon to water vapor at hightemperature for several hours.

To remove the thin film stent in this example, in Step C the nitinolthin film can be separated from the substrate by shearing the siliconoxide/silicon substrate boundary (rapid cooling or heating will initiatea thermal shock that will crack or shear the composite at the boundary).This will result in a composite of the nitinol thin film 42, the siliconoxide 50, and a very thin layer of silicon substrate 36. In Step D, thethin film is converted into the free standing stent by removing thesilicon oxide layer by etching the intermediate composite in a solutionof hydroflouric acid until the silicon oxide is dissolved and thesilicon has fallen away. The nitinol thin film remains as a freestanding thin film after the silicon oxide has been etched away. Thehydroflouric acid used in this process will also serve to etch or pickleaway any nitinol oxides from the nitinol. (Alternative release layerscan be used. The release layer may be comprised of silicon nitrides,which can be etched away with a CF4/O2 plasma etch without damaging thenitinol thin film. (Where aluminum is used as the substrate, aluminumoxide may be permitted or encouraged to form on the substrate, and thesubstrate may be dissolved away with a solution of sodium hypochlorideand sodium hydroxide. Aluminum without an oxide layer may be used, anddissolved away with a variety of acids and bases (hydrochloric acid,phosphoric acid, acetic acid, nitric acid). Where salts are used as thesubstrate, they may be dissolved in water.

Another method for creating a releasable thin film is to use thefollowing method for sputtering: A nitinol target of approximately 50atomic percent titanium. (depending on the desired transitiontemperature in the finished stent) is used. A substrate of silicon,glass copper, aluminum, kapton film or other material is locatedanywhere from 2 to 8 inches from the target. The substrate is preferablyoxidized, through an oxidation step designed to enhance the oxide layer(this step may comprise merely exposing the substrate to air). Thechamber atmosphere is set at about 1 millitor and cathode power is setfrom about 250 to 400 watts. The substrate is maintained or permitted toremain at chamber temperature, which should be slightly elevated overambient air temperature (normal room temperature of about 22°). The filmmay then be lifted from the substrate merely by laying an adhesive stripover the film and pulling the film off the substrate. (Those in thesputter deposition art will appreciate that the typical step ofchemically etching the substrate to remove the oxide layer need not beperformed.)

The stent film may be annealed to obtain the desired materialcharacteristics of the nitinol alloy, namely the shape memory,superelastic and pseudoelastic behaviors. The annealing may beaccomplished before or after removal of the stent sheet from thesubstrate. Annealing the stent sheet can be accomplished according tostandard annealing methods, which typically required a period of severalminutes at temperatures around 500° centigrade. Where the substratemelts at the annealing temperature (as is the case with plastics), itmay be removed either before the annealing process, or it may be meltedaway during the annealing process. The annealing process may helpseparate the thin film from the substrate where it is undertaken priorto release of the thin film from the substrate.

The perforation pattern on the stent sheet may be created with severalmethods. The perforations may be photochemically etched into the nitinolfilm after formation of the film into stent sheet, or the release layermay be etched and undercut with subsequent sputter deposition onto theremaining top surface of the release layer.

The process of photochemical machining may be used to cut theperforations in a solid sheet of nitinol thin film. Photochemicaletching may be accomplished either before or after the nitinol thin filmis lifted from the substrate. As shown in FIG. 5, in step A the thinfilm stent sheet 2 is still mounted on the substrate 36, and the sheetis coated with a layer of photoresist 51. In step B, the photoresist isprocessed (imaged and developed) to remove the coating in areascorresponding to the desired perforations, thereby creating a partialcoating which is a reverse image of the desired perforation pattern onthe sheet, and essentially a copy of the desired final perforated stentsheet. In step C, this intermediate composite is then soaked with anetchant for the nitinol which preferably does not react with theunderlying substrate or oxide layer. Suitable etchants includehydrofluoric acid, and BOE, HNO₃, H₂O solution. In step D, after theetchant has removed the stent sheet material in the perforation areas,the photoresist is removed with a wash of a suitable etchant whichdissolves the photoresist (there are numerous photoresists andcorresponding etchants. The remaining composite is the same as theinitial composite, except that the nitinol thin film is now perforated.In Step E, the release layer 50 is removed and the nitinol film is now afree standing stent perforated as desired.

The process of forming the thin film stent with perforations may bemodified so that the perforations are created during the depositionprocess. Rather than etch the finished thin film to create theperforations, the substrate may be coated with a waste layer (using aphotoresist compound or other etchable compound) which is etched with areverse image of the stent, and the stent film may be deposited over thereverse image waste layer. The process is a lift off technique similarto techniques used in microprocessor fabrication.

The process is illustrated in FIG. 6. At step A, the substrate 36,removable oxide layer 50 are coated with a waste layer 52. The wastelayer may be comprised of a photoresist compound or other readilyremovable material. If made of a photoresist, the waste layer is imaged(exposed and developed) with a reverse image of the desired stent. Atstep B, the waste layer has numerous perforations 53 which correspond tothe actual structure of the stent, and numerous strips and coated areas54 which correspond to the perforations of the stent. The side walls 55of the coated areas are undercut (to this end, the waste layer may becomprised of two layers, a top layer which is imaged and developed,followed by non-directional etching or wet etching of the underlyinglayer). The process creates a reverse slope photoresist side-wallprofile. In step C, the stent material is sputtered with a line of sightdeposition technique which avoids coating the side walls 55 (in contrastto step coverage or conformal coverage). The result is a layer of thinfilm of nitinol 56 on the substrate and a thin film of nitinol 57 on thewaste layer. The film 56 on the substrate is the stent, formed asdesired with perforations, struts, staves, and border areas as dictatedby the reverse image in the photoresist applied in step B. In step D,the waste material is dissolved and removed, along with waste film 57sputter on top of the waste material. In step E, the silicon oxide layer50 is dissolved away to remove the fully formed stent from thesubstrate.

Various photoresists may be used as the waste material, and numerousother compounds may be suitable. Where the photoresist is AZ-5214-E orequivalent, it may be removed in the final step with an ultrasoniccleaning in a bath of acetone. The waste layer may also be made ofsilicon nitride, removed with a phosphoric acid etch. The material“lifted off” is the waste nitinol deposited on the areas correspondingto the perforations in step D. The portion of the film left on thesubstrate is also lifted off, but preserved as the desired end produce.

The lift off process places the waste nitinol material over the wastelayer 52, and sputters the desired film onto the substrate. The processmay be reversed as illustrated in FIG. 7 as follows: At step A, thesubstrate 36 and removable oxide layer 50 are coated with a waste layer52. The waste layer may be comprised of a photoresist compound or otherreadily removable material. If made of a photoresist, the waste layer isimaged (exposed and developed) with positive (rather that a reverse)image of the desired stent. At step B, the waste layer has numerousperforations 58 which correspond to the actual perforations of thestent, and numerous strips and coated areas 59 which correspond to theactual structural elements of the stent. In step C, the stent materialis sputtered with a line of sight deposition technique which avoidscoating the side walls 55. The result is a layer of thin film of nitinol60 on the substrate and a thin film of nitinol 61 on the waste layer.The film 60 on the substrate is the wasted thin film stent, while thefilm 61 on the waste layer is the stent, formed as desired withperforations, struts, staves, and border areas as dictated by thepositive image in the photoresist applied in step B. In step D, thephotoresist is dissolved away in a bath of acetone or other solvent. Inthis process, the material “lifted off” is the desired stent. In eitherprocess of lift off or reverse lift off, the photoresist may be apositive (exposed areas become soluble) or negative photoresist (exposedareas become insoluble) which may be exposed with a positive or reverseimage of the desired stent. Thus, where the waste perforations arelifted off (leaving a positive on the substrate), and the waste materialis a positive photoresist, the photoresist is exposed in areascorresponding to the stent structure (rather than the perforations) andthe photoresist is developed to remove the exposed areas. Where thewasted stent material is lifted off and the waste material is a negativephotoresist, the photoresist is exposed in areas corresponding to thestent perforations (rather than the structure) and the photoresist isdeveloped to remove the unexposed areas. Where the waste perforationsare deposited on the substrate and the actual stent is deposited on thewaste material, and the waste material is a positive photoresist, thephotoresist is exposed in areas of perforation and the photoresist isdeveloped to removed exposed areas. Where the waste perforations aredeposited on the substrate and the actual stent is deposited on thewaste material, and the waste material is a negative photoresist, thenthe photoresist is exposed in areas of stent structure and thephotoresist is developed to remove unexposed areas.

The strength and resilience of the stent may be improved with hotisostatic processing (HIP or hipping) of the stent to remove voids inthe sputtered stent and increase the density of the stent. In thisprocess, after the sputtered stent has been released from the substrateand etched to form the perforation patterns, it is placed in a HIPchamber for exposure to a high pressure, high temperature atmosphere ofinert gas (argon) to eliminate any voids in the thin film. Temperaturesup to 2000° C. and pressures up to 650 MPa (94,000 psi) may be appliedin the process, and the pressure and temperature may be applied forvariable time periods of minutes to hours, depending on how long ittakes for the voids in the stent to fill. After hipping, the thin filmcan be additionally strengthened by cold working, for example by packrolling the thin film in one or more passes through a thin film roller.(Since hipping generally increases fatigue life of the hipped article,is may beneficially be used with all stents, implanted prostheses andother medical devices subject to repetitive stress. It may also bebeneficially applied to sputtered nitinol actuators generally, such asthe micro-valves and micro-grippers mentioned in the background.)

The process of forming a rolled sheet stent according to the descriptionmay be varied in many particulars. While we have illustrated the processwith the example of a nitinol stent, any metal and many polymers can besputtered into the thin film. While we have illustrated the broaderinvention of creation of a free standing thin film medical device withthe example of a stent, various other devices may be created with theprocess. While we have used the specific example of physical vapordeposition through sputtering, the many forms of thin film depositionmay be used. We expect that more desirable substrates, release layers,waste layers and target compositions may be discovered and applied inthe making stents and medical devices according to the inventive methodspresented above. Thus, while the preferred embodiments of the devicesand methods have been described in reference to the environment in whichthey were developed, they are merely illustrative of the principles ofthe inventions. Other embodiments and configurations may be devisedwithout departing from the spirit of the inventions and the scope of theappended claims.

1. A method for manufacturing a body implantable thin film medicaldevice, said method comprising: providing a substrate; depositing a thinfilm of material onto the substrate; releasing the thin film of materialfrom the substrate; and forming the thin film of material into a medicaldevice.
 2. The method of claim 1 wherein: the substrate comprises amaterial selected from the group consisting of highly polished metal,fluorite, silicon, glass, kapton film, a plastic, or a salt; and thethin film of material comprises a material selected from the groupconsisting of a cobalt-chromium-nickel alloy, tantalum, stainless steel,a metal or a polymer.
 3. The method of claim 1 wherein the step offorming the thin film of material into a medical device comprisesforming the thin film of material into a stent.
 4. The method of claim 3comprising the further step of providing the stent with a plurality ofperforations.
 5. The method of claim 4 wherein the step of providing thestent with a plurality of perforations comprises providing a pluralityof partially sinusoidally curved perforations.
 6. The method of claim 4wherein the step of providing the stent with a plurality of perforationscomprises providing a plurality of crescent-shaped perforations.
 7. Themethod of claim 4 wherein the step of providing the stent with aplurality of perforations comprises providing the stent with a pluralityof arcuate-shaped perforations.
 8. The method of claim 4 wherein thestep of providing the stent with a plurality of perforations comprisesproviding the stent with a plurality of slots.
 9. The method of claim 4wherein the step of providing the stent with a plurality of perforationscomprises providing the stent with a plurality of perforations that areabout 0.03 inches to about 0.07 inches apart radially and about 0.02634inches apart longitudinally, and each perforation is about 0.00625inches wide and about 0.06102 inches long.
 10. The method of claim 1wherein the step of forming the thin film of material into a medicaldevice comprises the step of forming the thin film of material into astent having a wrap length of about 6 millimeters to about 75millimeters and a bridge length of about 2 millimeters to about 20millimeters.
 11. The method of claim 1 wherein the step of forming thethin film of material into a medical device comprises the step offorming the thin film of material into a rolled stent that, whenunrolled, has a diameter of about 1 millimeter to about 6 millimeters.